Am I going to be okay with 48v DTM mono blocks into Maggie .7?
because they have independent power supplies for each channel. You can calculate the maximum volume with the output power and efficiency....
I think in feet. For the sake of easy calculation, I'll make the assumption that 3 feet is close enough to 1m. However, 1m is actually 3.28 feet.
Mag .7 efficiency is rated at 86dB 2.83V/1m. 2.83V is used since 2.83V (RMS) into 8Ω is 1W. Power is not used for sensitivity ratings because response is based on a fixed voltage, not a fixed power. If power was fixed, the voltage used for testing would need to be varied over frequency based on the change in impedance -- that gets unnecessarily messy, so the standard is voltage. Anyway, we need to know the impedance to calculate how much power 2.83V requires. In this case, it's 2W because the .7's impedance is 4Ω NOMINAL. I suspect it's closer to 3Ω. It takes more power to drive 3Ω than 4Ω. 1.33 times the power (!), actually, but we can adjust for the difference later.
Normal listening is more like 2m away, so you lose 6dB from dispersion. However, 2 speakers (stereo) gives you 3 more dB. So, we start with 6 feet from 2 speakers at 2W (each) for a net loss of 3dB for 83dB.
That will be important in just a bit....
Now, we double the power iteratively until we get close to the amplifier power, adding 3dB each time (not exactly, but very close)....
4W for +3dB
8W for +6dB
16W for +9dB
32W for +12dB
64W for +15dB
128W for +18dB
256W for +21dB
512W for +24dB
1024W for +27dB
Since the 48V STM
outputs roughly 120Wpc up to
8Ω (due to the power supply's limits, so even into 2Ω, it's still 120Wpc max), and that's close enough to 128W, we get +18dB for a total of 101dB SPL
. Please note that's MAX!
Since the 48V DTM
outputs roughly 240Wpc into 4 Ω, and that's close enough to 256W, we get +21dB for a total of 104dB SPL
In both cases, the power supply output power limits the SPL in more than one way. Since the load with LRS or .7 Mags is likely closer to 3Ω, using a 60V power supply enables the DTM or STM to output more power into 3Ω than 4Ω, so.... With the STM, the power supply drives two channels, so I recommend the 60V power supply for an STM driving Mags
. Since DTMs are monoblocks, that issue goes away....60V DTMs
output >530W into 3Ω. That adds more than 3dB for >107dB SPL
MAX in this case.Here's a handy calculator for SPL: